This invention relates to a process for producing monoalkyl aromatic compounds by alkylation. Specifically, this invention relates to highly-selective alkylation and transalkylation to produce cumene and ethylbenzene.
Alkylation of aromatic compounds with a C2 to C4 olefin and transalkylation of polyalkylaromatic compounds are two common reactions for producing monoalkyl aromatic compounds. Examples of these two reactions that are practiced industrially to produce ethylbenzene are the alkylation of benzene with ethylene and the transalkylation of benzene and a diethylbenzene. A simplified summary of the alkylation reaction and its common product and by-products is given below: 
Although the formation of the diethylbenzene and triethylbenzene isomers might, at first glance, be viewed as by-products that represent a reduction in the efficient utilization of ethylene, in fact each can be readily transalkylated by benzene to produce ethylbenzene, as shown below: 
Combining alkylation and transalkylation can thus maximize ethylbenzene production. Such a combination can be carried out in a process having two reaction zones, one for alkylation and the other for transalkylation, or in a process having a single reaction zone in which alkylation and transalkylation both occur. In many cases, a single reaction zone is preferred over two reaction zones because of the savings in capital investment.
One disadvantage of alkylation-transalkylation processes, regardless of whether the alkylation and transalkylation reactions occur in the same or separate reaction zones, is that by-product 1,1-diphenylethane (1,1-DPE) can not be converted to ethylbenzene by alkylation or transalkylation, and thus 1,1-DPE represents a reduction in ethylene utilization efficiency and a loss of ethylene. In fact, the by-production of 1,1-DPE, as well as of the heavier polyethylated benzenes other than diethylbenzene and triethylbenzene, represents virtually all of the reduction in the ethylene utilization efficiency and a loss of benzene as well. As used herein, the term xe2x80x9cheaviesxe2x80x9d refers to polyalkyl aromatics other than dialkyl and trialkyl aromatics and to other even heavier alkylation and transalkylation by-products including diphenylalkanes (DPA) and alkylated diphenylalkanes (DPAs), such as diphenylethane (DPE), alkylated diphenylethanes (DPEs), diphenylpropane (DPP), and alkylated diphenylpropanes (DPPs). The current minimum requirement for combination processes is that 1,1-DPE be not more than 1.0 wt-% relative to ethylbenzene. The formation of 1,1-DPE is assuming added importance and significance in view of the expectation in some areas of near-term minimum standards for the content of 1,1-DPE of not more than 0.5 wt-%.
In reaction zones where alkylation and transalkylation occur, it is known that the formation of 1,1-DPE depends in part on two key operating variables. The first operating variable is the molar ratio of phenyl groups per ethyl group, which is often referred to herein as the phenyl/ethyl ratio. The numerator of this ratio is the number of moles of phenyl groups passing through the reaction zone during a specified period of time. The number of moles of phenyl groups is the sum of all phenyl groups, regardless of the compound in which the phenyl group happens to be. In the context of ethylbenzene production, for example, one mole of benzene, one mole of ethylbenzene, and one mole of diethylbenzene each contribute one mole of phenyl group to the sum of phenyl groups. The denominator of this ratio is the number of moles of ethyl groups passing through the reaction zone during the same specified period of time. The number of moles of ethyl groups is the sum of all ethyl and ethenyl groups, regardless of the compound in which the ethyl or ethenyl group happens to be, except that paraffins are not included. Paraffins, such as ethane, propane, n-butane, isobutane, pentanes, and higher paraffins are excluded from the computation of the number of moles of ethyl groups. For example, one mole of ethylene and one mole of ethylbenzene each contribute one mole of ethyl group to the sum of ethyl groups, whereas one mole of diethylbenzene contributes two moles of ethyl groups and one mole of triethylbenzene contributes three moles of ethyl groups.
The second operating variable that affects the 1,1-DPE formation is the concentration of ethylene in the alkylation zone. A practical, mathematical approximation is that the concentration of ethylene depends on the molar ratio of phenyl groups per ethyl group according to the formula:
[ethylene]≈[phenyl/ethyl ratio]xe2x88x921.
Thus, increasing the phenyl/ethyl ratio decreases the concentration of ethylene.
It is known that a low concentration of ethylene or a high molar ratio of phenyl groups per ethyl group minimizes formation of 1,1-DPE. The amount of 1,1-DPE formed depends on the phenyl/ethyl ratio according to the formula:
[1,1-DPE]≈[phenyl/ethyl ratio]xe2x88x922.
Thus, increasing the phenyl/ethyl ratio decreases the amount of 1,1-DPE formed. Although the decrease in 1,1-DPE formation that is conferred by a small increase in phenyl/ethyl ratio may be small, it also is very significant, resulting in a high phenyl/ethyl ratio being the condition of choice for minimizing 1,1-DPE formation. However, a high phenyl/ethyl ratio increases capital and operating costs that are usually associated with the recovery of excess benzene. These costs give impetus to a search for an ethylbenzene process that minimizes 1,1-DPE formation at a low phenyl/ethyl ratio.
In the prior art, the search for a commercially-viable alkylation process that not only produces a small amount of 1,1-DPE but also operates at a low phenyl/ethyl ratio in the alkylation zone has not been fruitful. All of the prior art processes follow the same, well-known approach of dividing the reaction zone into more and more catalyst beds and injecting smaller and smaller portions of the total ethylene into each bed. Where the allowed concentration of 1,1-DPE is relatively high, this approach undoubtedly confers some benefits. For example, if benzene is alkylated with ethylene in a single-bed alkylation zone that operates at a phenyl/ethyl molar ratio of 5, then the highest concentration of ethylene, which occurs at the point of ethylene injection, is 16.7 mol-%. Downstream of the ethylene injection point, the ethylene concentration decreases to very low concentrations as ethylene is consumed and ethylbenzene is formed, while the phenyl/ethyl ratio remains essentially the same. However, if the single bed is divided into four beds in series and if one-fourth of the required ethylene is injected into each bed, then the phenyl/ethyl ratio is 20 in the first bed, 10 in the second bed, 6.7 in the third bed, and 5 in the fourth bed. Accordingly, the highest concentration of ethylene is 4.8 mol-% in the first bed, 4.5 mol-% in the second bed, 4.3 mol-% in the third bed, and 4.2 mol-% in the fourth bed. Thus, dividing the bed and splitting the ethylene injection increases the phenyl/ethyl ratio and decreases the highest ethylene concentration.
But, in order to operate at the low phenyl/ethyl ratios and to also attain the low concentrations of 1,1-DPE that are expected to become the minimum standard in the near future, this prior art approach is not viable. For example, if benzene is alkylated with ethylene in a four-bed alkylation zone that operates at an overall phenyl/ethyl molar ratio of 2 rather than 5 as in the previous example, then the phenyl/ethyl ratio ranges from 8 in the first bed to 2 in the fourth bed, and the highest ethylene concentration ranges from 11.1 mol-% in the first bed to 8.3 mol-% in the fourth bed. Compared to the previous example, the ethylene concentration in each bed approximately doubled, which would result in an unacceptable amount of 1,1-DPE formation. In order to reduce the ethylene concentrations to those in the previous example, the number of beds would have to be increased from 4 to 10, simply as a consequence of the fact that the overall phenyl/ethyl ratio had decreased from 5 to 2.
While U.S. Pat. No. 5,877,370 (Gajda) describes that a reduction in the amount of 1,1-DPE formed in the production of ethylbenzene by alkylation of benzene with ethylene can be effected by alkylating at a low concentration of ethylene, the lowest concentration of ethylene that U.S. Pat. No. 5,877,370 teaches is 1.88 wt-% (Example 6, Table 1). The highest ratio of weight of recycle effluent per weight of fresh benzene that U.S. Pat. No. 5,877,370 teaches is 3 (Example 10, Table 3) which, at a phenyl/ethyl molar ratio of 5.0, corresponds to a ratio of weight of recycle effluent per weight of fresh feed (i.e., fresh benzene and fresh olefin) of 2.5.
Thus, in response to industry""s demands for lower phenyl/ethyl ratios and the market""s demand for lower ethylene concentrations, the prior art process inexorably divides the reaction zone into a large number of very small catalyst beds. Because of a variety of technical, economic, and practical considerations, this inefficient solution by the prior art processes is unacceptable in the hydrocarbon processing industry.
A method has been discovered to significantly reduce the formation of diphenylalkanes and/or to significantly decrease the catalyst deactivation rate in the production of alkyl aromatics such as ethylbenzene and cumene by alkylation using solid catalysts such as zeolite beta. This invention is particularly useful at a low molar ratio of phenyl groups per alkyl group (e.g., phenyl/ethyl ratio or pheny/propyl ratio). This invention passes an aromatic feed stream and an olefinic feed stream to an alkylation catalyst bed, where the ratio of the weight of the olefin entering the alkylation catalyst bed in the olefinic feed stream per unit time to the sum of the weights of compounds entering the alkylation catalyst bed per the same unit time, multiplied by 100, is less than 1.88. This invention uses a dilute ratio of olefin entering the alkylation catalyst bed and/or a dilute concentration of olefin in the alkylation catalyst bed to decrease the diphenylalkane formation and/or to decrease the catalyst deactivation rate. This dilution can be achieved by recycling a portion of the effluent from an alkylation reaction zone. This result using a solid catalyst such as zeolite beta was surprising and was not predictable from the prior art, which teaches that, in the production of ethylbenzene, for example, 1,1-DPE formation can be reduced only by increasing the phenyl/ethyl ratio or by increasing the number of catalyst beds. Moreover, prior art processed-using Y zeolite produce more 1,1-DPE and deactivate more rapidly as a result of using the same components and streams that confer benefits when using solid alkylation catalysts such as zeolite beta. Thus, a process of alkylating benzene with ethylene at a low ethylene concentration shows a significant selectivity advantage over one operating at a high ethylene concentration. By using this invention, ethylbenzene processes can now minimize 1,1-DPE formation and/or catalyst deactivation even when operating at low molar ratios of phenyl groups per ethyl group. With the problems of 1,1-DPE formation and/or catalyst deactivation now solved by this invention, ethylbenzene processes can now operate more profitably at a low molar ratio of phenyl groups per ethyl group.
Without limiting this invention to any particular theory, two working hypotheses may describe the underlying chemistry responsible for the observed results. One hypothesis is that in the alkylation of an aromatic by an olefin, when the concentration of an olefin decreases, there is a selective decrease in the reaction between the olefin and the alkyl aromatic. The products of this reaction are an alkyl aromatic and a paraffin that correspond to the olefin. The alkyl aromatic can in turn serve as an active alkylating agent and react with the aromatic to form by-product diarylalkane. Applying this hypothesis to the alkylation of benzene with ethylene, the apparently anomalous formation of 1,1-DPE would result from the following reaction: 
Where a catalyst is used, it is believed that the ethylbenzene and the styrene are chemisorbed on the catalyst, and that hydrogen transfer occurs from the ethylbenzene to ethylene. In any event, a decrease in the concentration of ethylene affords a decrease in the formation of styrene and in turn that of 1,1-DPE. The second working hypothesis that may describe the underlying chemistry is that, when the concentration of an olefin decreases, there is a selective decrease in the reaction between olefins. The product of this reaction is an oligomer, such as a dimer, which can react with an aromatic in a manner similar to that in which an undimerized olefin react with an aromatic. The products of these two reactions are two alkyl aromatics, one that corresponds to the dimer and another that corresponds to the olefin. These two alkyl aromatics can in turn react with each other to form by-product diarylalkane. Thus, applying this hypothesis to the alkylation of benzene with ethylene, the apparently anomalous formation of 1,1-DPE would result from the following reaction: 
The outcome of either foregoing hypothesis and its logical consequence is that one can expect a decrease in the olefin concentration to confer benefits generally upon the alkylation of aromatics with olefins.
This invention minimizes 1,1-DPE formation and/or solid catalyst deactivation by using one or more components or portions of the reaction zone effluent stream to prevent the ethylene concentration from ever attaining the high ethylene concentrations that are present in prior art processes. It is generally known that in prior art processes the concentration of ethylene in the reaction zone decreases from a relatively high concentration at the inlet point where ethylene is introduced to a relatively low concentration at the outlet where nearly all of the ethylene has been consumed. So, even in the prior art processes, low concentrations of ethylene can occur, especially near the outlet of the reaction zone. However, it has been discovered that even the localized high ethylene concentrations that occur in prior art processes at the point of ethylene injection produce unacceptably high concentrations of 1,1-DPE and/or unacceptably rapid deactivation rates. Thus, it is now recognized that one or more components or portions of the reaction zone effluent stream can preclude localized high ethylene concentrations and minimize 1,1-DPE formation and/or solid catalyst deactivation. Moreover, it has been recognized that aliquot portions of the reaction zone effluent are preferred over other portions of the reaction zone effluent and that selective choice of an aliquot portion of the reaction zone effluent can decrease not only the deactivation rate of solid catalyst but also the formation of other undesirable by-products besides 1,1-DPE, without the requirement that the molar ratio of phenyl groups per ethyl group be decreased.
The working hypotheses explain the formation of other diarylalkanes that correspond to other olefins alkylating other aromatics. For example, in the alkylation of benzene with propylene to produce cumene, the corresponding diarylalkane would probably be 2,2-diphenylpropane (2,2-DPP). Although formation of 1,1-diphenylpropane (1,1-DPP) is also possible, 2,2-DPP formation is believed to be more probable due to preferential reaction at the secondary carbon of the propylene.
It is a broad object of this invention to improve the selectivity of and to decrease the costs of processes for the alkylation of aromatics with olefins and the transalkylation of aromatics with polyalkyl aromatics. It is a specific object of this invention to minimize the formation of 1,1-diphenylethane (1,1-DPE) and the rate of catalyst deactivation in alkylation processes that produce ethylbenzene. It is a specific object of this invention to produce an alkylation effluent stream that contains less than 1.0 wt-% diarylalkane relative to the desired monoalkyl aromatic product. It is another specific object of this invention to decrease costs associated with operating alkylation processes by decreasing the molar ratio of phenyl groups per alkyl group at alkylation conditions.
In a broadembodiment, this invention is a process for producing a monoalkyl aromatic. An aromatic feed stream comprising a feed aromatic and an olefinic feed stream comprising an olefin pass to an alkylation catalyst bed in an alkylation reaction zone. The alkylation catalyst bed contains a solid catalyst. The ratio of the weight of the olefin entering the alkylation catalyst bed in the olefinic feed stream per unit time to the sum of the weights of compounds entering the alkylation catalyst bed per the unit time, multiplied by 100, is less than 1.88. The feed aromatic is alkylated with the olefin in the alkylation catalyst bed at alkylation conditions and in the presence of the solid catalyst to form a monoalkyl aromatic. The monoalkyl aromatic has one more alkyl group corresponding to the olefin than the feed aromatic. An effluent stream comprising the monoalkyl aromatic is withdrawn from the alkylation reaction zone. The monoalkyl aromatic is recovered from the effluent stream.
Other embodiments of this invention are described in the detailed description.
Prior art alkylation processes are well described in the literature.
U.S. Pat. No. 4,051,191 describes catalysts, reaction conditions, and a separation method for the recovery of cumene that uses a rectification zone and a two-column fractionation train.
U.S. Pat. Nos. 4,695,665 and 4,587,370 are particularly directed to the separation of products and the recovery of recycle streams from processes for the alkylation of aromatic hydrocarbons, and U.S. Pat. No. 4,695,665 discloses the use of a flash drum in combination with an effluent rectifier to recover unreacted feed components.
U.S. Pat. No. 4,876,408 discloses an alkylation process that uses zeolite beta having carbon deposits thereon to suppress alkylation activity and increase selectivity to monoalkylate.
U.S. Pat. No. 4,891,458 describes the use of beta zeolite for the alkylation of aromatic hydrocarbons with alkenes to produce alkyl aromatics. U.S. Pat. No. 4,891,458 also discloses that transalkylation can occur in an alkylation reactor, and that additional monoalkyl aromatic hydrocarbons can be produced in an alkylation reactor by recycling polyalkyl aromatic hydrocarbons to the alkylation reactor to undergo transalkylation. The teachings of U.S. Pat. No. 4,891,458 are incorporated herein by reference.
U.S. Pat. No. 4,922,053 describes a process for alkylating benzene with ethylene in a multibed reactor wherein polyethylbenzenes are recycled to the first alkylation bed and also to one or more of the other alkylation beds in order to increase ethylbenzene yield.
U.S. Pat. No. 5,030,786 discloses an alkylation process wherein the feed stream is dehydrated to enhance the performance of beta or Y zeolites in the alkylation process.
U.S. Pat. No. 5,227,558 discloses an alkylation process for the production of ethylbenzene that uses a steam modified zeolite beta catalyst.
U.S. Pat. No. 5,336,821 describes the use of beta zeolite for the alkylation of aromatic hydrocarbons in a process that is improved by an indirect heat exchanger to recover the heat of reaction. In one embodiment, the alkylation reactor effluent passes through the heat exchanger and is recycled to the alkylation reactor.
Prior art transalkylation processes are well described in the literature. U.S. Pat. No. 4,083,886 describes a process for the transalkylation of the alkyl aromatic hydrocarbons that uses a zeolitic catalyst. U.S. Pat. No. 4,891,458 describes the use of beta zeolite for the transalkylation of aromatic hydrocarbons with polyalkyl aromatic hydrocarbons. European Patent Application EP 0 733 608 A1 discloses the use of an alumina silicate catalyst having an average crystal size of less than about 0.5 microns for the transalkylation of polyalkyl benzenes with benzene in a reaction zone with an alkylating agent such, as ethylene.
Combination processes that produce alkyl aromatic products by using an alkylation reaction zone and a transalkylation reaction zone are also well known.
U.S. Pat. No. 4,008,290 describes a combination process in which the alkylation effluent and the transalkylation effluent are passed to a common separation zone, which separates the two effluents into product, by-product, and recycle streams including a recycle benzene stream. A portion of the alkylation effluent is recycled to the alkylation reaction zone in order to decrease the portion of the recycle benzene stream that is recycled to the alkylation reaction zone. The teachings of U.S. Pat. No. 4,008,290 are incorporated herein by reference.
U.S. Pat. No. 5,003,119 describes a combination process for producing monoalkyl aromatics in which the alkylation effluent passes to the transalkylation reaction zone, and the transalkylation effluent passes to a separation zone. U.S. Pat. No. 5,003,119 also describes passing dialkyl aromatics to the alkylation reaction zone. In addition, U.S. Pat. No. 5,003,119 teaches that diphenylalkanes are alkylation by-products and that a Zeolitic catalyst can be used to convert diphenylalkanes into lighter aromatics.
U.S. Pat. No. 5,177,285 discloses an alkylation process that is improved by maintaining the feed to the alkylation zone in a relatively wet condition and the feed to the transalkylation zone in a relatively dry condition. The process operates with a relatively pure ethylene feed as an alkylating agent with a large excess of benzene.
German Patent Application DE 19,516,717 discloses the preparation of 1,1 diphenylethanes by the addition reaction of benzene with styrene in the presence of beta zeolite.
A paper entitled xe2x80x9cDevelopment and Commercialization of Solid Acid Catalysts,xe2x80x9d by M. F. Bentham et al., was presented at the DGMK meeting on xe2x80x9cCatalysis of Solid Acids and Basesxe2x80x9d on Mar. 14-15, 1996, in Berlin, Germany, and describes the formation of diphenylethane in ethylbenzene processes by the reaction of benzene and ethylene to form styrene and ethane and by the reaction of benzene and styrene to form diphenylethane.
U.S. Pat. No. 5,902,917 describes a process for producing alkylaromatics, especially ethylbenzene and cumene, wherein a feedstock is first fed to a transalkylation zone and the entire effluent from the transalkylation zone is then cascaded directly into an alkylation zone along with an olefin alkylating agent, especially ethylene or propylene.
U.S. Pat. No. 5,998,684 describes a process for producing alkylaromatics that operates with an alkylation zone and a transalkylation zone, where the transalkylation zone and the alkylation zone are arranged for series flow and the transalkylation zone effluent is passed with an aromatic containing feed and the olefinic feed, which is preferably propylene or ethylene, to the alkylation zone.